In Virginia, there appear to be 390 exits (highway=motorway_junction) in OSM as of March 26, 2011, 1 of which is for a weigh station and 2 for rest areas. Aside from missing exits, there is also some cleanup needed. There is a mix of ref=* and name=* usage, though exit numbers should always go on the ref tag. There don't appear to be any exits tagged with exit_to=* to give descriptive text of the exit.
VDOT may be willing to provide a database of all the interstate exits in Virginia for merging with OSM data. There are 1094 listed exits, though some are for rest areas, scales, collector roads, etc.
How to merge
This data would probably best be merged with OSM by converting the Shapefile to OSM XML, then using JOSM and its merge utility] with each pair of exits. This should work whether or not exit information is tagged in OSM, and will take advantage of JOSM's conflict dialog.
Fields to be used:
- EXIT_NBR: The exit number, like 166
- EXIT_LTR: The exit number letter suffix. (X if it's an exit to collector roads (i.e. two or more exits off another road)
- LASTUPDATE: Date of last update, like 2006/04/20
Fields for reference only:
- EXIT_TYPE: One of Exit (983), Collector Roads (47, always accompanied by EXIT_LTR=X), Maintenance (3), Rest Area (41), Scales (12), Truck Inspection or Truck Escape (6), Wayside (Overlook) (2)
- RTE_NBR: Route number of highway exit is on
- RTE_DIR: Direction of highway exit is on
Fields that aren't useful:
- OBJECTID: internal id
- RTE_SYS_ID: Simply a concatenation of RTE_NBR, RTE_DIR, EXIT_NBR, and EXIT_LTR
- JURIS_CD: jurisdiction code?
- JURIS_NM: name of jurisdiction, for internal use
- EDITORNAME: name of user who created or updated entry